Electrical and Optical Properties of Rutile Single Crystals

Abstract

Clear, synthetic rutile (Ti${\mathrm{O}}_{2}$) single crystals have been investigated by electrical and optical methods. It seems possible to correlate the high temperature conductivity (${E}_{G}=3.05$ ev) with the threshold of optical absorption at low temperatures (3.03 ev) and with the maximum of the photoconductivity (3.03-3.06 ev). This evidence indicates an energy gap $\mathrm{ca} 3.05$ ev for rutile as an insulator.Semiconducting rutile, prepared by hydrogen reduction at temperatures \ensuremath{\le}800\ifmmode^\circ\else\textdegree\fi{}C, shows a blue color arising from an optical absorption maximum at $\mathrm{ca} 1.7$ \ensuremath{\mu} (0.73 ev). Conductivity-temperature plots for slightly reduced specimens indicate an optical activation energy of 0.68 ev. A theoretical calculation for the ionization of the first electron from an oxygen vacancy indicates 0.74 ev as the expected value, in good agreement with the experimental results. At room temperature the mobility of electrons in slightly reduced single crystals is $\mathrm{ca} {10}^{\ensuremath{-}4}$ ${\mathrm{m}}^{2}$ /v-sec.Strongly reduced rutile is opaque; a comparison of electron concentrations calculated from weight loss and Hall coefficient data shows that for samples in which the electron concentration is ${10}^{26}$ /${\mathrm{m}}^{3}$, all contribute to conduction at room temperature.